The present invention relates to a lamination type secondary battery, and in particular, to a lamination type secondary battery, which comprises a plurality of positive electrodes and negative electrodes laminated on each other with a separator between them, and current collecting tabs of the positive electrodes are connected in parallel to current collecting tabs of the negative electrodes.
Various types of secondary batteries are known. As a battery having high energy density, efforts are being made on practical utilization of nonaqueous electrolyte battery such as lithium ion battery.
As the nonaqueous electrolyte battery, two types of batteries are known: a cylindrical type battery, which comprises a cylindrical battery element accommodated in a battery case, and the cylindrical battery element being produced by winding up positive and negative electrodes each in band-like shape with a separator between them. There is also a rectangular type battery, which comprises a battery element produced by winding up the electrodes as described above, the battery element is then molded in flat shape and is accommodated in a rectangular battery case. These are now widely used as power source for portable type devices such as personal computer, portable telephone, etc.
Besides the battery using the positive electrodes and the negative electrodes in band-like shape, a lamination type secondary battery is known, which comprises a plurality of positive electrodes and negative electrodes in planar shape, the positive electrodes and the negative electrodes being laminated via a separator, and current collecting tabs bonded to each of the electrodes are connected in parallel.
In the secondary battery comprising wound-up type battery element using the positive electrodes and the negative electrodes in band-like shape, the current collecting tabs are bonded to the negative electrodes and the positive electrodes, and these are laminated in the order of separatorxe2x80x94negative electrodexe2x80x94separatorxe2x80x94positive electrode, and the battery element can be produced. This contributes to easier manufacture of the battery element, while there have been problems in that these components may be bent or crooked during winding operation, or that the battery active material layer tends to be thicker on the portion where radius of curvature is smaller, or that electric current is often concentrated at a certain point. Also, in the wound-type battery element, a force is applied in a direction opposite to the winding direction, and it is necessary to arrange so that the wound-type battery element is not unwound. Further, in the wound-type battery element, when a plurality of current collecting tabs are mounted on the electrodes to charge or discharge large electric current, the shape of the wound-type element may be distorted, or problems may occur in the connection between the current collecting tab and the terminal for external connection.
In contrast, in the lamination type secondary battery, which comprises a plurality of electrodes in planar shape laminated on each other, deformation caused by volume change of the active material during charging or discharging takes place only in laminating direction. Thus, the influence of the volume change on the battery is low. Also, it is easy to accurately connect the current collecting tabs mounted on the electrodes of the battery element. Therefore, it can be applied in extensive range from small-size battery with smaller current capacity to large-size battery, in which charging and discharging of large electric current can be achieved. If the current collecting tab having larger conductive area is used, charging and discharging of larger electric current can be easily performed. In this sense, the lamination type secondary battery seems to be very promising as a structure for large-size battery.
FIG. 10 is a perspective view to explain a conventional lamination type secondary battery.
A lamination type secondary battery 30 comprises a plurality of positive electrodes 31 and negative electrodes 32, and these electrodes are arranged at face-to-face position via a separator 33 between them. Positive electrode current collecting tabs 34 and negative electrode current collecting tabs 35 bonded to the positive electrodes and the negative electrodes respectively are connected in parallel. Then, the tabs are conductively connected to a positive electrode terminal 37 and a negative electrode terminal 38 mounted on a lid 36 of the battery. Then, these are accommodated in a battery case 39, and the opening between the battery case 39 and the lid 36 is sealed by means such as laser welding.
However, for the purpose of laminating a plurality of positive electrodes and negative electrodes in planar shape with a separator between them in the lamination type secondary battery, it is essential to laminate and assemble these electrodes only after accurate positioning so that position of each of the electrode plates is not deviated from each other. This means that much labor and time are required in the positioning and assembling processes of the electrodes.
Also, in case of lithium ion battery, the negative electrodes having larger surface areas than the positive electrodes must be used to prevent deposition of metal lithium on the surface of the negative electrode, which is caused by intensive concentration of electric current during charging operation. It is also indispensable to prevent the deposition of metal lithium due to intensive concentration of electric current on the ends of the negative electrodes. This can be accomplished by providing the active material of the negative electrode to all portions facing to the positive electrode.
Therefore, in the lithium ion battery, it is very important to perform accurate positioning and to laminate the positive electrodes and the negative electrodes different in size on each other. Deviation in positions of the electrodes not only adversely affects performance characteristics of the battery but also it may result in unfavorable condition such as short-circuiting with the positive electrode due to the deposition of metal lithium.
Further, in the lamination type secondary battery, current collecting tabs mounted on the positive electrodes and the negative electrodes are bonded to a conductive connection terminal mounted on the battery lid. Then, the battery lid is engaged in the opening of the battery case. In this connection, the current collecting tab must be longer in length compared with the distance between the electrode and the conductive connection terminal when the battery lid is engaged in the opening. When a longer current collecting tab is used, short-circuiting may occur due to the contact of the current collecting tabs with different polarities with each other or due to contact between the current collecting tab and the electrode having different polarities. In particular, in the battery used as a mobile power source, there is high possibility that short-circuiting may occur due to vibration or shock from outside.
To prevent the short-circuiting with the current collecting tab, a method may be adopted to cover the current collecting tab with an insulating material. This leads to the increase in the number of assembling processes to cover the current collecting tab with the insulating material. Also, this causes relative decrease of the quantity of the battery active material in the battery case. For a battery with high energy density, there have been many problems in taking these measures.
It is an object of the present invention to provide a lamination type secondary battery such as a lithium ion battery, which comprises positive electrodes and negative electrodes different in size, these electrodes being laminated via a separator, and by which it is possible to perform accurate positioning of the positive electrode and the negative electrodes and to provide a battery, which has high capacity density, and there is no possibility of short-circuiting in the battery, which is easier to assemble.
The present invention provides a lamination type secondary battery, which comprises positive electrodes and negative electrodes having different surface areas, said positive electrodes and said negative electrodes being laminated face-to-face with a separator therebetween, each of the electrodes having smaller surface area is covered with a separator main body and arranged at the center and positioned with equal spacing from outer periphery of the separator main body, and outer periphery of the electrode having larger surface area has a size equal to the size of outer periphery of the separator main body to cover the electrode having smaller surface area.
Further, the present invention provides the lamination type secondary battery as described above, wherein a current collecting tab provided on the electrode having smaller surface area is covered with a protective member integrally formed with the separator main body.
Also, the present invention provides the lamination type secondary battery as described above, wherein when the electrode having smaller surface area is projected perpendicularly to the electrode having larger surface area, the entire portion of the projected area is within the electrode having larger surface area.
Further, the present invention provides the lamination type secondary battery as described above, wherein when the electrode having smaller surface area is projected perpendicularly to the opposing electrode having large surface area, an area outside the projected portion is positioned face-to-face to the current collecting tab mounted on the electrode having smaller surface area and is provided with a notched portion larger than the width of the current collecting tab.
Also, the present invention provides the lamination type secondary battery as described above, wherein at least a part of the separator positioned in an area outside the electrode having smaller surface area is thermally fused.
Further, the present invention provides the lamination type secondary battery as described above, wherein the electrode having smaller surface area is designed in polygonal shape, and each corner of the electrode is rounded.
Also, the present invention provides the lamination type secondary battery as described above, wherein external form of the electrode is designed with curved surface.